Philips scn2681 DATASHEETS

INTEGRATED CIRCUITS

SCN2681

Dual asynchronous receiver/transmitter
(DUART)

Product specification
Supersedes data of 1995 May 01
IC19 Data Handbook

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1998 Sep 04

Philips Semiconductors Product specification

DESCRIPTION

SCN2681Dual asynchronous receiver/transmitter (DUART)

DESCRIPTION

The Philips Semiconductors SCN2681 Dual Universal
Asynchronous Receiver/Transmitter (DUART) is a single-chip
MOS-LSI communications device that provides two independent
full-duplex asynchronous receiver/transmitter channels in a single
package. It interfaces directly with microprocessors and may be
used in a polled or interrupt driven system.

The operating mode and data format of each channel can be
programmed independently. Additionally, each receiver and
transmitter can select its operating speed as one of eighteen fixed
baud rates, a 16X clock derived from a programmable counter/timer,
or an external 1X or 16X clock. The baud rate generator and
counter/timer can operate directly from a crystal or from external
clock inputs. The ability to independently program the operating
speed of the receiver and transmitter make the DUART particularly
attractive for dual-speed channel applications such as clustered
terminal systems.

Each receiver is quadruply buffered to minimize the potential of
receiver over-run or to reduce interrupt overhead in interrupt driven
systems. In addition, a flow control capability is provided to disable
a remote DUART transmitter when the buffer of the receiving device
is full.

Also provided on the SCN2681 are a multipurpose 7-bit input port
and a multipurpose 8-bit output port. These can be used as general
purpose I/O ports or can be assigned specific functions (such as
clock inputs or status/interrupt outputs) under program control.

The SCN2681 is available in three package versions: 40-pin and
28–pin, both 0.6” wide DIPs; a compact 24-pin 0.4” wide DIP; and a
44-pin PLCC.

FEA TURES

•Dual full-duplex asynchronous receiver/transmitter

•Quadruple buffered receiver data registers

•Programmable data format

– 5 to 8 data bits plus parity
– Odd, even, no parity or force parity
– 1, 1.5 or 2 stop bits programmable in 1/16-bit increments

•Programmable baud rate for each receiver and transmitter

selectable from:
– 22 fixed rates: 50 to 115.2k baud

•16-bit programmable Counter/Timer

– Non-standard rates to 115.2Kb
– One user-defined rate derived from programmable

timer/counter

– External 1X or 16X clock

•Parity, framing, and overrun error detection

•False start bit detection

•Line break detection and generation

•Programmable channel mode

– Normal (full-duplex)
– Automatic echo
– Local loopback
– Remote loopback

•Multi-function programmable 16-bit counter/timer

•Multi-function 7-bit input port

– Can serve as clock or control inputs
– Change of state detection on four inputs
– 100kΩ typical pull-up resistor

•Multi-function 8-bit output port

– Individual bit set/reset capability
– Outputs can be programmed to be status/interrupt signals

•Versatile interrupt system

– Single interrupt output with eight maskable interrupting

conditions

– Output port can be configured to provide a total of up to six

separate wire-ORable interrupt outputs

•Maximum data transfer: 1X – 1MB/sec, 16X – 125kB/sec

•Automatic wake-up mode for multidrop applications

•Start-end break interrupt/status

•Detects break which originates in the middle of a character

•On-chip crystal oscillator

•Single +5V power supply

•Commercial and industrial temperature ranges available

•DIP and PLCC packages

ORDERING INFORMATION

NOTES:

1. 400mil-wide Dual In-Line Package

2. 600mil-wide Dual In-Line Package

1998 Sep 04 853–1077 19970

ORDER CODE

Commercial Industrial

VCC = +5V +5%, TA = 0°C to +70°C VCC = +5V +10%, TA = -40°C to +85°C

Plastic DIP Plastic LCC Plastic DIP Plastic LCC

1

24-Pin

2

28-Pin

2

40-Pin
44-Pin Not available SCN2681AC1A44 Not available SCN2681AE1A44

SCN2681AC1N24 Not available SCN2681AE1N24 Not available
SCN2681AC1N28 Not available SCN2681AE1N28 Not available
SCN2681AC1N40 Not available SCN2681AE1N40 Not available

2

Philips Semiconductors Product specification

SCN2681Dual asynchronous receiver/transmitter (DUART)

PIN CONFIGURATIONS

A0

IP3

A1

IP1

A2

A3

IP0

WRN

RDN
RXDB
TXDB

OP1

OP3

OP5

OP7

D1
D3
D5

D7

GND

1
2
3

4
5

6
7
8
9

10

DIP

11
12

13

14
15
16
17
18

19

40
39
38

37
36

35
34
33
32
31
30
29

28

27
26
25
24
23

22
2120

V

CC

IP4
IP5

IP6
IP2

CEN
RESET
X2
X1/CLK
RXDA
TXDA
OP0

OP2

OP4
OP6
D0
D2
D4

D6
INTRN

WRN

RDN

RXDB

TXDB

OP1

1

A0

2

A1

3

A2

4

A3

5

6
7

DIP

8
9

10

D1

11

D3

12

D5

13

D7

14

28
27
26
25
24

23
22

21
20
19
18
17

16
15

V

CC

IP2
CEN

RESET
X2

X1/CLK
RXDA

TXDA
OP0
D0
D2
D4

D6
INTRNGND

A3

WRN

RDN

RXDB

TXDB

D1

D5

GND

1

A1

2

A2

3

4

5

6

DIP

7

8

9

D3

10

11

D7

12

24

A0

23

V

CC

22

CEN

21

RESET

20

X1/CLK

19

RXDA

18

TXDA

17

D0

16

D2
D4

15

14

D6

13

INTRN

INDEX

CORNER

PIN/FUNCTION PIN/FUNCTION

1NC 23NC

6

7

17

18

1

PLCC

TOP VIEW

40

39

29

28

2 A0 24 INTRN
3 IP3 25 D6
4A1 26D4
5 IP1 27 D2
6A2 28D0
7 A3 29 OP6
8 IP0 30 OP4
9 WRN 31 OP2
10 RDN 32 OP0
11 RXDB 33 TXDA
12 NC 34 NC
13 TXDB 35 RXDA
14 OP1 36 X1/CLK
15 OP3 37 X2
16 OP5 38 RESET
17 OP7 39 CEN
18 D1 40 IP2
19 D3 41 IP6
20 D5 42 IP5
21 D7 43 IP4

22 GND 44 V

CC

SD00084

Figure 1. Pin Configurations

1998 Sep 04

3

Philips Semiconductors Product specification

SYMBOL

TYPE

NAME AND FUNCTION

SCN2681Dual asynchronous receiver/transmitter (DUART)

PIN DESCRIPTION

APPLICABLE

40/44 28 24

D0–D7 X X X I/O Data Bus: Bidirectional 3-State data bus used to transfer commands, data and status be-

CEN X X X I Chip Enable: Active-Low input signal. When Low, data transfers between the CPU and the

WRN X X X I Write Strobe: When Low and CEN is also Low, the contents of the data bus is loaded into

RDN X X X I Read Strobe: When Low and CEN is also Low, causes the contents of the addressed regis-

A0–A3 X X X I Address Inputs: Select the DUART internal registers and ports for read/write operations.
RESET X X X I Reset: A High level clears internal registers (SRA, SRB, IMR, ISR, OPR, OPCR), puts

INTRN X X X O Interrupt Request: Active-Low, open-drain, output which signals the CPU that one or more

X1/CLK X X X I Crystal 1: Crystal connection or an external clock input. A crystal of a clock the appropriate

X2 X X I Crystal 2: Crystal connection. See Figure 7. If a crystal is not used it is best to keep this pin

RxDA X X X I Channel A Receiver Serial Data Input: The least significant bit is received first. “Mark” is

RxDB X X X I Channel B Receive Serial Data Input: The least significant bit is received first. “Mark” is

TxDA X X X O Channel A Transmitter Serial Data Output: The least significant bit is transmitted first.

TxDB X X X O Channel B Transmitter Serial Data Output: The least significant bit is transmitted first.

OP0 X X O Output 0: General purpose output or Channel A request to send (RTSAN, active-Low). Can

OP1 X X O Output 1: General purpose output or Channel B request to send (RTSBN, active-Low). Can

OP2 X O Output 2: General purpose output or Channel A transmitter 1X or 16X clock output, or Chan-

OP3 X O Output 3: General purpose output or open-drain, active-Low counter/timer output or Channel

OP4 X O Output 4: General purpose output or Channel A open-drain, active-Low, RxRDYA/FFULLA

OP5 X O Output 5: General purpose output or Channel B open-drain, active-Low, RxRDYB/FFULLB

OP6 X O Output 6: General purpose output or Channel A open-drain, active-Low, TxRDYA output.
OP7 X O Output 7: General purpose output or Channel B open-drain, active-Low, TxRDYB output.
IP0 X I Input 0: General purpose input or Channel A clear to send active-Low input (CTSAN). Pin

IP1 X I Input 1: General purpose input or Channel B clear to send active-Low input (CTSBN). Pin

IP2 X X I Input 2: General purpose input or counter/timer external clock input. Pin has an internal V

IP3 X I Input 3: General purpose input or Channel A transmitter external clock input (TxCA). When

IP4 X I Input 4: General purpose input or Channel A receiver external clock input (RxCA). When the

tween the DUART and the CPU. D0 is the least significant bit.

DUART are enabled on D0-D7 as controlled by the WRN, RDN and A0-A3 inputs. When
High, places the D0-D7 lines in the 3-State condition.

the addressed register. The transfer occurs on the rising edge of the signal.

ter to be presented on the data bus. The read cycle begins on the falling edge of RDN.

OP0–OP7 in the High state, stops the counter/timer, and puts Channels A and B in the inac­tive state, with the TxDA and TxDB outputs in the mark (High) state. Clears Test modes, sets
MR pointer to MR1.

of the eight maskable interrupting conditions are true.

frequency (nominally 3.6864 MHz) must be supplied at all times. For crystal connections see
Figure 7, Clock Timing.

not connected although it is permissible to ground it.

High, “space” is Low.

High, “space” is Low.

This output is held in the “mark” condition when the transmitter is disabled, idle or when oper­ating in local loopback mode. “Mark” is High, “space” is Low.

This output is held in the “mark” condition when the transmitter is disabled, idle or when oper­ating in local loopback mode. “Mark” is High, “space” is Low.

be deactivated automatically on receive or transmit.

be deactivated automatically on receive or transmit.

nel A receiver 1X clock output.

B transmitter 1X clock output, or Channel B receiver 1X clock output.

output.

output.

has an internal V

has an internal V

pull-up device supplying 1 to 4 A of current.

the external clock is used by the transmitter, the transmitted data is clocked on the falling
edge of the clock. Pin has an internal V

external clock is used by the receiver, the received data is sampled on the rising edge of the
clock. Pin has an internal V

pull-up device supplying 1 to 4 A of current.

CC

pull-up device supplying 1 to 4 A of current.

CC

pull-up device supplying 1 to 4 A of current.

CC

pull-up device supplying 1 to 4 A of current.

CC

CC

1998 Sep 04

4

Philips Semiconductors Product specification

SYMBOL

TYPE

NAME AND FUNCTION

SYMBOL

PARAMETER

TEST CONDITIONS

UNIT

SCN2681Dual asynchronous receiver/transmitter (DUART)

PIN DESCRIPTION (Continued)

APPLICABLE

40/44 28 24

IP5 X I Input 5: General purpose input or Channel B transmitter external clock input (TxCB). When

IP6 X I Input 6: General purpose input or Channel B receiver external clock input (RxCB). When the

V

CC

X X I Power Supply: +5V supply input.

GND X X I Ground

the external clock is used by the transmitter, the transmitted data is clocked on the falling
edge of the clock. Pin has an internal V

pull-up device supplying 1 to 4 A of current.

CC

external clock is used by the receiver, the received data is sampled on the rising edge of the
clock. Pin has an internal V

pull-up device supplying 1 to 4 A of current.

CC

ABSOLUTE MAXIMUM RATINGS

SYMBOL

T
T

A

STG

Operating ambient temperature range
Storage temperature range -65 to +150 °C
All voltages with respect to ground

1

PARAMETER RATING UNIT

2

3

See Note 4 °C

-0.5 to +6.0 V

NOTES:

1. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and
functional operation of the device at these or any other condition above those indicated in the operation section of this specification is not
implied.

2. For operating at elevated temperatures, the device must be derated based on +150

o

C maximum junction temperature.

3. This product includes circuitry specifically designed for the protection of its internal devices from damaging effects of excessive static
charge. Nonetheless, it is suggested that conventional precautions be taken to avoid applying any voltages larger than the rated maxima.

4. Parameters are valid over specified temperature range. See Ordering information table for applicable operating temperature range and V
supply range.

DC ELECTRICAL CHARACTERISTICS

1, 2, 3

T

= -40°C to +85°C, V

A

= +5.0V  10%

CC

CC

LIMITS

Min Typ Max

V
V
V

I

IL

I

LL

I

X1L

I

X1H

I

X2L

I

X2H

I

OC

I

OCC

V

IL

V

IH

V

IH

V

IH

OL
OH
OH

Input high voltage (except X1/CLK)
Input high voltage (except X1/CLK)

Output high voltage (except o.d. outputs)

Output high voltage (except o.d. outputs)

Input leakage current
Data bus 3-stage leakage current
X1/CLK low input current

X1/CLK high input current

X2 low input current
X2 high input current
Open-collector output leakage current
Power supply current

Input low voltage

Input high voltage (X1/CLK)

Output low voltage

5
4

I

= 2.4mA

5

4

OL

I

= -400µA

OH

I

= -400µA

OH

VIN = 0 to V

VO = 0.4 to V

VIN = 0, X2 grounded

CC

CC

VIN = 0, X2 floated

V

= VCC, X2 grounded

IN

VIN = VCC, X2 floated

VIN = 0, X1/CLK floated

VIN = VCC, X1/CLK floated

VO = 0.4 to V

0°C to +70°C version

CC

2

2.5
4

2.4

2.9

-10

-10

-4

-3

-1
0

-100
0

-10

-2

-1.5

0.2

3.5

-30

+30

-40°C to +85°C version

0.8

0.4

10
10

0
0
1

10

0

100

10
150
175

V
V
V
V
V
V
V

µA
µA

mA
mA
mA
mA

µA
µA
µA

mA
mA

NOTES:

1. Parameters are valid over specified temperature range. See Ordering information table for applicable operating temperature range and V
supply range.

CC

2. All voltage measurements are referenced to ground (GND). For testing, all inputs except X1/CLK swing between 0.4V and 2.4V with a
transition time of 20ns maximum. For X1/CLK this swing is between 0.4V and 4.4V . All time measurements are referenced at inpu t voltages
of 0.8V and 2.0V as appropriate.

3. Typical values are at +25°C, typical supply voltages, and typical processing parameters.

< 0°C

4. T

A

5. T

> 0°C

A

1998 Sep 04

5

Philips Semiconductors Product specification

SYMBOL

PARAMETER

UNIT

SCN2681Dual asynchronous receiver/transmitter (DUART)

AC CHARACTERISTICS T

= -40°C to +85°C1, V

A

= +5.0V  10%

CC

2, 3, 4, 5

LIMITS

Min Typ Max

Reset Timing (Figure 3)

t

RES

Bus Timing (Figure 4)

t

AS

t

AH

t

CS

t

CH

t

RW

t

DD

t

DF

t

DS

t

DH

t

RWD

Port Timing (Figure 5)

t

PS

t

PH

t

PD

RESET pulse width 200 ns

6

A0-A3 setup time to RDN, WRN Low 10 ns
A0-A3 hold time from RDN, WRN Low 100 ns
CEN setup time to RDN, WRN Low 0 ns
CEN hold time from RDN, WRN High 0 ns
WRN, RDN pulse width 225 ns
Data valid after RDN Low 175 ns
Data bus floating after RDN High 100 ns
Data setup time before WRN High 100 ns
Data hold time after WRN High 20 ns
High time between READs and/or WRITE

6

7, 8

200 ns

Port input setup time before RDN Low 0 ns
Port input hold time after RDN High 0 ns
Port output valid after WRN High 400 ns

Interrupt Timing (Figure 6)

t

IR

INTRN (or OP3-OP7 when used as interrupts) negated from:

Read RHR (RxRDY/FFULL interrupt) 300 ns
Write THR (TxRDY interrupt) 300 ns
Reset command (delta break interrupt) 300 ns
Stop C/T command (counter interrupt) 300 ns
Read IPCR (input port change interrupt) 300 ns
Write IMR (clear of interrupt mask bit) 300 ns

Clock Timing (Figure 7)

t

CLK

f

CLK

t

CTC

f

CTC

9

t

RX

9

f

RX

9

t

TX

9

f

TX

10

X1/CLK High or Low time 100 ns
X1/CLK frequency 2.0 3.6864 4.0 MHz
CTCLK (IP2) High or Low time 100 ns
CTCLK (IP2) frequency 0 4.0 MHz
RxC High or Low time 220 ns
RxC frequency (16X)

(1X)

0
0

2.0

1.0

MHz

MHz
TxC High or Low time 220 ns
TxC frequency (16X)

(1X)

0
0

2.0

1.0

MHz

MHz

Transmitter T iming (Figure 8)

9

t

TXD

t

TCS

9

TxD output delay from TxC external clock input on IP pin 350 ns
Output delay from TxC low at OP pin to TxD data output 0 150 ns

Receiver Timing (Figure 10)

9

t

RXS

t

RXH

9

RxD data setup time before RxC high at external clock input on IP pin 240 ns
RxD data hold time after RxC high at external clock input on IP pin 200 ns

NOTES:

1. For operating at elevated temperatures, the device must be derated based on +150°C maximum junction temperature.

2. Parameters are valid over specified temperature range.

3. All voltage measurements are referenced to ground (GND). For testing, all inputs except X1/CLK swing between 0.4V and 2.4V with a
transition time of <

20ns. For X1/CLK this swing is between 0.4V and 4.4V . All time measurements are referenced at input voltages of 0.8V

and 2.0V as appropriate.

4. Typical values are at +25°C, typical supply voltages, and typical processing parameters.

5. Test condition for outputs: C

6. Timing is illustrated and referenced to the WRN and RDN inputs. The device may also be operated with CEN as the ‘strobing’ input. In this

= 150pF, except interrupt outputs. Test condition for interrupt outputs: CL = 50pF, R

L

= 2.7kΩ to VCC.

L

case, all timing specifications apply referenced to the falling and rising edges of CEN, CEN and RDN (also CEN and WRN) are ANDed
internally . As a consequence, the signal asserted last initiates the cycle and the signal negated first terminates the cycle.

7. If CEN is used as the ‘strobing’ input, the parameter defines the minimum High times between one CEN and the next. The RDN signal must
be negated for t

8. Consecutive write operations to the same command register require at least three edges of the X1 clock between writes.

to guarantee that any status register changes are valid.

RWD

9. This parameter is not applicable to the 28-pin device.

10.Operation to 0MHz is assured by design. However, operation at low frequencies is not tested and has not been characterized.

1998 Sep 04

6

Philips Semiconductors Product specification

SCN2681Dual asynchronous receiver/transmitter (DUART)

BLOCK DIAGRAM

D0–D7

RDN

WRN

CEN

A0–A3

RESET

INTRN

8

BUS BUFFER

OPERATION CONTROL

ADDRESS

4

DECODE

R/W CONTROL

INTERRUPT CONTROL

IMR
ISR

TIMING

BAUD RATE

GENERATOR

CLOCK

SELECTORS

CONTROL

TIMING

INTERNAL DATABUS

CHANNEL A

TRANSMIT

HOLDING REG

TRANSMIT

SHIFT REGISTER

RECEIVE

HOLDING REG (3)

RECEIVE

SHIFT REGISTER

MRA1, 2

CRA
SRA

CHANNEL B
(AS ABOVE)

INPUT PORT

CHANGE OF

STATE

DETECTORS (4)

IPCR

ACR

TxDA

RxDA

TxDB

RxDB

7

IP0-IP6

COUNTER/

TIMER

X1/CLK

X2

XTAL OSC

CSRA

CSRB

ACR

CTUR

CTLR

Figure 2. Block Diagram

BLOCK DIAGRAM

The SCN2681 DUART consists of the following eight major sections:
data bus buffer, operation control, interrupt control, timing,
communications Channels A and B, input port and output port.
Refer to the block diagram.

Data Bus Buffer

The data bus buffer provides the interface between the external and
internal data buses. It is controlled by the operation control block to
allow read and write operations to take place between the controlling
CPU and the DUART.

OUTPUT PORT

FUNCTION

SELECT LOGIC

OPCR

OPR

8

OP0-OP7

V

CC

GND

SD00085

Operation Control

The operation control logic receives operation commands from the
CPU and generates appropriate signals to internal sections to
control device operation. It contains address decoding and read and
write circuits to permit communications with the microprocessor via
the data bus buffer.

Interrupt Control

A single active-Low interrupt output (INTRN) is provided which is
activated upon the occurrence of any of eight internal events.
Associated with the interrupt system are the Interrupt Mask Register

1998 Sep 04

7

Philips Semiconductors Product specification

SCN2681Dual asynchronous receiver/transmitter (DUART)

(IMR) and the Interrupt Status Register (ISR). The IMR may be
programmed to select only certain conditions to cause INTRN to be
asserted. The ISR can be read by the CPU to determine all
currently active interrupting conditions.

Outputs OP3-OP7 can be programmed to provide discrete interrupt
outputs for the transmitter, receivers, and counter/timer.

Timing Circuits

The timing block consists of a crystal oscillator, a baud rate
generator, a programmable 16-bit counter/timer, and four clock
selectors. The crystal oscillator operates directly from a 3.6864MHz
crystal connected across the X1/CLK and X2 inputs. If an external
clock of the appropriate frequency is available, it may be connected
to X1/CLK. The clock serves as the basic timing reference for the
Baud Rate Generator (BRG), the counter/timer, and other internal
circuits. A clock signal within the limits specified in the
specifications section of this data sheet must always be supplied to
the DUART.

If an external clock is used instead of a crystal, both X1 and X2
should use a configuration similar to the one in Figure 7.

The baud rate generator operates from the oscillator or external
clock input and is capable of generating 18 commonly used data
communications baud rates ranging from 50 to 38.4k baud. The
clock outputs from the BRG are at 16X the actual baud rate. The
counter/timer can be used as a timer to produce a 16X clock for any
other baud rate by counting down the crystal clock or an external
clock. The four clock selectors allow the independent selection, for
each receiver and transmitter, of any of these baud rates or external
timing signal.

Counter/Timer (C/T)

The counter timer is a 16 bit programmable divider that operates
one of three modes: Counter, T imer or Time Out mode. In all three
modes it uses the 16-bit value loaded to the CTUR and CTLR
registers. (Counter timer upper and lower preset registers).

•In the timer mode it generates a square wave.

•In the counter mode it generates a time delay.

•In the time out mode it monitors the receiver data flow and signals

data flow has paused. In the time out mode the receiver controls
the starting/stopping of the C/T.

The counter operates as a down counter and sets its output bit in
the ISR (Interrupt Status Register) each time it passes through 0.
The output of the counter/timer may be seen on one of the OP pins
or as an Rx or Tx clock.

The Timer/Counter is controlled with six (6) “commands”; Start C/T,
Stop C/T, write C/T, preset registers, read C/T value, set or reset
time out mode.

Please see the detail of the commands under the Counter/Timer
register descriptions.

Communications Channels A and B

Each communications channel of the SCN2681 comprises a
full-duplex asynchronous receiver/transmitter (UART). The
operating frequency for each receiver and transmitter can be
selected independently from the baud rate generator, the counter
timer, or from an external input.

The transmitter accepts parallel data from the CPU, converts it to a
serial bit stream, inserts the appropriate start, stop, and optional
parity bits and outputs a composite serial stream of data on the TxD

output pin. The receiver accepts serial data on the RxD pin,
converts this serial input to parallel format, checks for start bit, stop
bit, parity bit (if any), or break condition and sends an assembled
character to the CPU.

The input port pulse detection circuitry uses a 38.4kHz sampling
clock derived from one of the baud rate generator taps. This results
in a sampling period of slightly more than 25µs (this assumes that
the clock input is 3.6864MHz). The detection circuitry, in order to
guarantee that a true change in level has occurred, requires two
successive samples at the new logic level be observed. As a
consequence, the minimum duration of the signal change is 25µs if
the transition occurs “coincident with the first sample pulse”. The
50µs time refers to the situation in which the change-of-state is “just
missed” and the first change-of-state is not detected until 25µs later.

Input Port

The inputs to this unlatched 7-bit port can be read by the CPU by
performing a read operation at address D16. A High input results in
a logic 1 while a Low input results in a logic 0. D7 will always read
as a logic 1. The pins of this port can also serve as auxiliary inputs
to certain portions of the DUART logic.

Four change-of-state detectors are provided which are associated
with inputs IP3, IP2, IP1 and IP0. A High-to-Low or Low-to-High
transition of these inputs lasting longer than 25 – 50µs, will set the
corresponding bit in the input port change register. The bits are
cleared when the register is read by the CPU. Any change-of-state
can also be programmed to generate an interrupt to the CPU.

All the IP pins have a small pull-up device that will source 1 to 4 A
of current from V

connections if they are not used.

V

CC

. These pins do not require pull-up devices or

CC

Output Port

The output port pins may be controlled by the OPR, OPCR, MR and
CR registers. Via appropriate programming they may be just
another parallel port to external circuits, or they may represent many
internal conditions of the UART. When this 8-bit port is used as a
general purpose output port, the output port pins drive a state which
is the complement of the Output Port Register (OPR). OPR(n) = 1
results in OP(n) = Low and vice versa. Bits of the OPR can be
individually set and reset. A bit is set by performing a write operation
at address E16 with the accompanying data specifying the bits to be
set (1 = set, 0 = no change).

Likewise, a bit is reset by a write at address F16 with the
accompanying data specifying the bits to be reset (1 = reset, 0 = no
change).

Outputs can be also individually assigned specific functions by
appropriate programming of the Channel A mode registers (MR1A,
MR2A), the Channel B mode registers (MR1B, MR2B), and the
Output Port Configuration Register (OPCR).

Please note that these pins drive both high and low. HOWEVER
when they are programmed to represent interrupt type functions
(such as receiver ready, transmitter ready or counter/timer ready)
they will be switched to an open drain configuration in which case an
external pull-up device would be required.

TRANSMITTER OPERATION

The SCN2681 is conditioned to transmit data when the transmitter is
enabled through the command register. The SCN2681 indicates to
the CPU that it is ready to accept a character by setting the TxRDY
bit in the status register. This condition can be programmed to

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Philips Semiconductors Product specification

SCN2681Dual asynchronous receiver/transmitter (DUART)

generate an interrupt request at OP6 or OP7 and INTRN. When a
character is loaded into the Transmit Holding Register (THR), the
above conditions are negated. Data is transferred from the holding
register to transmit shift register when it is idle or has completed
transmission of the previous character. The TxRDY conditions are
then asserted again which means one full character time of buffering
is provided. Characters cannot be loaded into the THR while the
transmitter is disabled.

The transmitter converts the parallel data from the CPU to a serial
bit stream on the TxD output pin. It automatically sends a start bit
followed by the programmed number of data bits, an optional parity
bit, and the programmed number of stop bits. The least significant
bit is sent first. Following the transmission of the stop bits, if a new
character is not available in the THR, the TxD output remains High
and the TxEMT bit in the Status Register (SR) will be set to 1.
Transmission resumes and the TxEMT bit is cleared when the CPU
loads a new character into the THR.

If the transmitter is disabled, it continues operating until the
character currently being transmitted is completely sent out. The
transmitter can be forced to send a continuous Low condition by
issuing a send break command.

The transmitter can be reset through a software command. If it is
reset, operation ceases immediately and the transmitter must be
enabled through the command register before resuming operation.
If CTS operation is enable, the CTSN input must be Low in order for
the character to be transmitted. If it goes High in the middle of a
transmission, the character in the shift register is transmitted and
TxDA then remains in the marking state until CTSN goes Low. The
transmitter can also control the deactivation of the RTSN output. If
programmed, the RTSN output will be reset one bit time after the
character in the transmit shift register and transmit holding register
(if any) are completely transmitted, if the transmitter has been
disabled.

Receiver

The SCN2681 is conditioned to receive data when enabled through
the command register. The receiver looks for a High-to-Low
(mark-to-space) transition of the start bit on the RxD input pin. If a
transition is detected, the state of the RxD pin is sampled each 16X
clock for 7 1/2 clocks (16X clock mode) or at the next rising edge of
the bit time clock (1X clock mode). If RxD is sampled High, the start
bit is invalid and the search for a valid start bit begins again. If RxD
is still Low, a valid start bit is assumed and the receiver continues to
sample the input at one bit time intervals at the theoretical center of
the bit, until the proper number of data bits and parity bit (if any)
have been assembled, and one stop bit has been detected. The
least significant bit is received first. The data is then transferred to
the Receive Holding Register (RHR) and the RxRDY bit in the SR is
set to a 1. This condition can be programmed to generate an
interrupt at OP4 or OP5 and INTRN. If the character length is less
than eight bits, the most significant unused bits in the RHR are set to
zero.

After the stop bit is detected, the receiver will immediately look for
the next start bit. However, if a non-zero character was received
without a stop bit (framing error) and RxD remains Low for one half
of the bit period after the stop bit was sampled, then the receiver
operates as if a new start bit transition had been detected at that
point (one-half bit time after the stop bit was sampled).

The parity error, framing error, overrun error and received break
state (if any) are strobed into the SR at the received character
boundary , before the RxRDY status bit is set. If a break condition is

detected (RxD is Low for the entire character including the stop bit),
a character consisting of all zeros will be loaded into the RHR and
the received break bit in the SR is set to 1. The RxD input must
return to high for two (2) clock edges of the X1 crystal clock for the
receiver to recognize the end of the break condition and begin the
search for a start bit. This will usually require a high time of one

X1 clock period or 3 X1 edges since the clock of the controller
is not synchronous to the X1 clock.

Receiver FIFO

The RHR consists of a First-In-First-Out (FIFO) stack with a
capacity of three characters. Data is loaded from the receive shift
register into the topmost empty position of the FIFO. The RxRDY bit
in the status register is set whenever one or more characters are
available to be read, and a FFULL status bit is set if all three stack
positions are filled with data. Either of these bits can be selected to
cause an interrupt. A read of the RHR outputs the data at the top of
the FIFO. After the read cycle, the data FIFO and its associated
status bits (see below) are ‘popped’ thus emptying a FIFO position
for new data.

Receiver Status Bits

In addition to the data word, three status bits (parity error, framing
error, and received break) are also appended to each data character
in the FIFO (overrun is not). Status can be provided in two ways, as
programmed by the error mode control bit in the mode register. In
the ‘character’ mode, status is provided on a character-by-character
basis; the status applies only to the character at the top of the FIFO.
In the ‘block’ mode, the status provided in the SR for these three bits
is the logical-OR of the status for all characters coming to the top of
the FIFO since the last ‘reset error’ command was issued. In either
mode reading the SR does not affect the FIFO. The FIFO is
‘popped’ only when the RHR is read. Therefore the status register
should be read prior to reading the FIFO.

If the FIFO is full when a new character is received, that character is
held in the receive shift register until a FIFO position is available. If
an additional character is received while this state exits, the
contents of the FIFO are not affected; the character previously in the
shift register is lost and the overrun error status bit (SR[4] will be
set-upon receipt of the start bit of the new (overrunning) character).

The receiver can control the deactivation of RTS. If programmed to
operate in this mode, the RTSN output will be negated when a valid
start bit was received and the FIFO is full. When a FIFO position
becomes available, the RTSN output will be re-asserted
automatically. This feature can be used to prevent an overrun, in
the receiver, by connecting the RTSN output to the CTSN input of
the transmitting device.

Receiver Reset and Disable

Receiver disable stops the receiver immediately – data being
assembled if the receiver shift register is lost. Data and status in the
FIFO is preserved and may be read. A re-enable of the receiver
after a disable will cause the receiver to begin assembling
characters at the next start bit detected.

A receiver reset will discard the present shift register data, reset the
receiver ready bit (RxRDY), clear the status of the byte at the top of
the FIFO and re-align the FIFO read/write pointers. This has the
appearance of “clearing or flushing” the receiver FIFO. In fact, the
FIFO is NEVER cleared! The data in the FIFO remains valid until
overwritten by another received character. Because of this,
erroneous reading or extra reads of the receiver FIFO will miss-align

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